1. Field of the Invention
This invention relates generally to a glow plug to be used for preheating an auxiliary combustion chamber or combustion chamber in a diesel engine, and more particularly to a glow plug for diesel engines comprising a ceramic heater having self-saturation properties that permits after glowing for long hours.
2. Description of Prior Art
In general, a diesel engine has poor starting properties at low temperatures. To assist the diesel engine in starting, therefore, a glow plug is usually provided in an auxiliary combustion chamber or combustion chamber to raise the intake air temperature or use as an ignition source the heat generated by applying electricity to the plug. The glow plug is usually of a sheathed heater type constructed by filling a metallic sheath with heat-resistant insulating powder and embedding a heater coil, made of ferrochrome, nickel, etc., in the powder. In addition to this, a ceramic heater type is also known, as disclosed in Japanese Patent Laid-open No. 41523/1982, which comprises a heating wire, made of tungsten, etc., is embedded in an insulating ceramic material, such as silicon nitride. The ceramic heater type has been widely used in recent years because it has a better heat transfer efficiency and an excellent heat generating performance since it becomes red hot in a short period of time during heating, compared with the sheathed heater type which involves indirect heating by means of the heat-resistant insulating powder and the sheath.
The glow plug of the ceramic heater type, however, has a metallic heating wire, made of tungsten, etc., embedded in the inside of an insulating ceramic material, such as silicon nitride. Because of different coefficients of thermal expansion of both the members, a sharp temperature rise during heating and the repeated use of the heater tend to cause a deterioration the durability of the ceramic heater, posing problems in the reliability, including heat resistance, of the ceramic heater. This also results in increased manufacturing costs.
To solve this problem, a ceramic heater construction in which an electrically conductive ceramic material having a that of an insulating ceramic material is used as a heating wire has been proposed in Japanese Patent Laid-open Nos. 9085/1985 and 14784/1985. Both the proposed glow plugs however, still have problems in terms of both construction and function, and therefore have not been put into commercial application.
That is, both of them have a number of problems, such as a poor quick-heating function, the difficulty in molding a complex construction of the electrode take-off portion, and prolonged after-glow time.
To overcome the above problems, the present applicant had previously filed patent applications for his inventions of glow plugs for diesel engines in which a ceramic heater consisting of a U-shaped electrically conductive ceramic material is bonded to, and supported by, the inside of a hollow holder (Japanese Patent Application Nos. 299338/1985, 299339/1985, 256354/1986, 256355/1986, and 2643/1987). Although these inventions could solve those problems inherent in the prior art, the bonded portion of the holder and the ceramic heater must have not only a sufficient bonding strength but also good electrical insulating properties.
To this end, an insulating layer is provided between the holder and the ceramic heater. However, there still are a number of problems to be solved, such as the unavailability of suitable materials having excellent mechanical strength and electrical insulating performance needed for the insulating layer, cracking due to the thermal stress caused by the difference in the thermal expansion coefficients of component materials, and improper bonding due to insufficient wetting properties of the members being bonded.
The most common method for improving insulation performance between the holder and the ceramic heater, is to increase the thickness of an insulating layer. Formation of an insulating layer of a large thickness in a single operation, however. tends to cause voids, microcracks and other defects in the layer, leading to deteriorated insulation performance, or insulation breakdown in extreme cases. Another method for forming an insulating layer is to apply an insulating layer to the surface using a brush. This method, however, involves the difficulty in providing a uniform layer over the surface. This results in local bonding failure between the heater and the holder.
As the material for bonding the holder and the ceramic heater, glass is most commonly used. The use of conventional amorphous glass, however, leads to the softening of the amorphous glass in the subsequent process of brazing the holder and the ceramic heater with silver solder (at approx. 750.degree. C). This could also deteriorate insulation performance and air-tightness.
As the material for the ceramic heater, electrically conductive sintered sialon is known. With the heater made of electrically conductive sintered sialon, part of the surface of the ceramic heater can be blistered when voltage is applied to the ceramic heater to cause self-heating. FIG. 1 is an enlarged cross section of part of the ceramic heater. A crack A is found developing around the blistered area in the direction parallel to the surface. Another crack B is also found in the direction normal to the surface. All this results in a loss in electrical conductivity, deteriorating the heating function of the ceramic heater.